1. Field of the Invention
The present invention relates to a motor control apparatus for a reel-to-reel tape drive system, more particularly to a magnetic tape motion control apparatus having a stop-lock means for an unbuffered reel-to-reel magnetic tape drive system which includes separate drive motors for each reel of a reel-to-reel magnetic tape transport apparatus.
2. Description of the Related Art
Recently, a magnetic tape motion control apparatus for a reel-to-reel magnetic tape drive system without a tape buffering mechanism, such as a capstan, has come into use for computer systems.
U.S. Pat. No. 3,764,087 and U.S. Pat. No. 3,984,868 disclose reel-to-reel magnetic tape motion control systems each employing separate drive motors for each reel and requiring no magnetic tape buffering or tachometer in the magnetic tape feed path. In each control system, the reel motors are selectively driven and a tachometer is associated with only the take up reel shaft to provide one pulse per revolution. The pulses are accumulated in a counter as a continuing count indicative of the radius of the magnetic tape wrapped around that reel. However, in these control systems, only one reel is driven at a time, and no means is installed to provide tension control. Accordingly, the above prior arts suffer from slippage of the magnetic tape and lack of precise control of magnetic tape tension and speed.
In order to overcome the above disadvantage, U.S. Pat. No. 4,125,881 discloses an improved magnetic motion control apparatus for a reel-to-reel magnetic tape drive system. The magnetic motion control apparatus is intended to allow high acceleration rates without magnetic tape slippage, maintain magnetic tape tension substantially constant throughout operation, maintain magnetic tape speed substantially constant between periods of acceleration and deceleration, maintain the magnetic tape position when called for, and control the acceleration rate independently of changes in the reel radii.
The above magnetic motion control apparatus includes a magnetic tape driving mechanism, a group of sensors, and a control circuit. The magnetic tape driving mechanism consists of a pair of motors operated jointly by means of a controller, set forth later, a pair of reels driven by the motors, a read/write magnetic head provided between the reels, and a movable rotor provided between the reels and adjacent to the read/write magnetic head. The sensors are a pair of tachometers each directly connected to a respective reel and detecting a rotation of the reel, and a tension sensor detecting a tension applied to the movable rotor. One tachometer outputs a higher rate of rotation pulses than another tachometer to derive radii of the reels. One tachometer also outputs two different phase signals for detecting a rotational direction of the reels. The controller outputs control signals to the pair of motors to achieve the above-mentioned objectives. The controller includes a circuit for providing a single gating pulse each time one of the reels rotates through a relatively large preselected angle, and a circuit for providing a plurality of pulses, one each time the other of the reels rotates through another angle which is substantially smaller than the preselected angle. The controller also includes a counter for counting the number of the plurality of pulses and a circuit operating in response to the single pulse to gate out the count from the counter. The controller further includes a circuit operating in response to each such gated out count to provide currents to each of the motors of a magnitude corresponding to the gated out count according to a predetermined servo algorithm for controlling rotation of the reels, thereby to provide continuing changes in currents as the radii of the reels change.
The prior art disclosed in U.S. Pat. No. 4,125,881 still suffers from inaccuracies in fine position control and fine tension control when the magnetic motion control apparatus is applied to a reel-to-reel magnetic tape drive system in which a high density magnetic tape is used.
Magnetic tape has been highly improved in storage density to, for example, 32,000 bits per inch (BPI) in a longitudinal direction and eighteen tracks, with a resultant shortening of the inter-record gap (IRG). This requires highly precise position control due to the transport direction and unevenness of characteristics of the separate drive motors. The high density magnetic tape enables shortening of the length of the magnetic tape to, for example, 800 feet (243.8 m), with a resultant reduction in the diameter of a reel to, for example, four inches (10.16 cm). This also means a small inertial value of the reel for winding the magnetic tape. The small inertial value means a short delay time in operating the reels, thus facilitating formation of a simplified control system only when the drive system operates in an ideal condition. On the other hand, the drive system having the small inertial value is susceptible to numerous disturbances, such as external noise, unevenness of the characteristics of the drive motors, and fluctuations in tension applied to the magnetic tape and transport speed of the magnetic tape.
Accordingly, in a reel-to-reel magnetic tape drive system in which high density magnetic tape is used, a fine control system is required as a countermeasure for the above disturbances.